The present invention relates to a method for forming a lubricating film on a surface of aluminum or aluminum alloy (hereinafter abbreviated as "aluminum etc.") at the time of subjecting said aluminum etc. to cold forging, drawing and the like.
Phosphate film is widely used as a lubricating film in cold forging or such of steel and nonferrous metals.
Formation of phosphate film on aluminum etc. has so far been performed by using, for example, a solution of the following composition: Zn.sup.2+ :0.7%, PO.sub.4.sup.3- :1.0%, NO.sub.3.sup.- :2.0%, BF.sub.4.sup.- :1.0%. However, when aluminum etc. are treated with this solution, the situation of film formation changes in a short period, so that it has been difficult to form a desired phosphate film in a stable manner. In order to form the phosphate film stably it has so far been practiced to renew the solution frequently. However, the disposal of the waste liquor of the solution, which contains fluorides, is troublesome.
An object of the present invention is to provide a method for stably forming a lubricating film suited to cold forging or such on aluminum etc. without the need of frequent renewal of treating solution. Further, the present invention discloses a method for forming a lubricating film on aluminum etc. by using a treating solution whose waste liquor can be easily disposed.
The present inventors have made extensive study to find the reason why the situation of phosphate film formation changes in a short period as described above in the previous method of lubricating treatment, and as a result have made the following findings.
i) When aluminum etc. are treated in a phosphating solution containing no fluoride the metal surface is coated by inactive oxide film or hydrated oxide film, which makes the formation of phosphate film largely impossible.
ii) When a fluoride is incorporated into the solution, the inactive oxide film and hydrated oxide film are activated by a fluoride and the film layer of phosphate is formed. At this time, however Al.sup.3+ ions dissolve out into the treating solution and remain therein.
iii) The Al.sup.3+ ions are gradually accumulated in the phosphating solution and come to inhibit the formation of phosphate film. Therefore, the solution containing accumulated Al.sup.3+ ions can hardly reproduce the situation of phosphate film formation as desired even when the concentrations of the other ingredients are adjusted by replenishment of chemicals.
As described above, a treating solution containing a fluoride is naturally used in order to form phosphate film directly on the surface of aluminum etc. However, with the treating solution containing a fluoride, the situation of phosphate film formation becomes unstable in a short period.
The present inventors have made further study on the method for forming phosphate film layer on aluminum etc. and as a resultantly attained the following findings.
iv) When aluminum etc. are subjected beforehand to chemical zinc deposition plating, phosphate film layer can be formed stably on the aluminum etc. irrespective of whether the phosphating solution contains fluorine or not.
v) When chemical zinc deposition plating is applied beforehand, no dissolution nor accumulation of Al.sup.3+ ions in the solution takes place, and the situation of phosphate film formation is maintained as desired in a stable manner even when the solution is repeatedly used over a long period.